The present disclosure relates to an image sensor and a method for manufacturing the same.
Image sensors are semiconductor devices which can convert optical images into electrical signals. Such image sensors can typically be classified as either a Charge Coupled Device (CCD) or a Complementary Metal Oxide Semiconductor (CMOS) Image Sensor (CIS).
During the fabrication of image sensors, a photodiode may be formed in a substrate using ion implantation. As the size of a photodiode is reduced for the purpose of increasing the number of pixels without increasing chip size, the area of a light receiving portion is also reduced, thereby resulting in a reduction in image quality.
Also, since a stack height does not reduce as much as the reduction in the area of the light receiving portion, the number of photons incident to the light receiving portion is also being reduced due to diffraction of light called Airy disk.
As an alternative to overcome this limitation, an attempt of forming a photodiode using amorphous silicon (Si), or forming a readout circuitry in a silicon (Si) substrate using a method such as wafer-to-wafer bonding, and forming a photodiode on and/or over the readout circuitry has been made (referred to as a “three-dimensional (3D) image sensor”). The photodiode is connected with the readout circuitry through a metal interconnection.
In the manufacture of a 3D image sensor according to a related-art, there are difficulties in performing a wafer-to wafer alignment between a photodiode positioned at an upper part of a chip and a readout circuit unit formed in a silicon substrate, and ensuring an ohmic contact due to a poor contact between an interconnection of the readout circuit unit and the photodiode.
Also, there is a limitation in that an image defect is generated by damage to an image sensing device such as an etching damage because a pixel isolation region is formed by using a device isolation layer in a related-art.
According to a related-art, there is a limitation in that a charge sharing phenomenon may occur because both the source and the drain of the transfer transistor are heavily doped with N-type impurities. The charge sharing phenomenon may cause reduction of the sensitivity of an output image and generation of image error. Also, photo charges may not smoothly move between a photodiode and a readout circuitry, causing generation of a dark current and reduction of saturation and sensitivity.